This invention relates generally to instantaneous overcurrent elements used in microprocessor-based protective relays for power systems and more specifically concerns such an overcurrent element which provides accurate current magnitude information under normal operating conditions and when the current waveform from the current transformer is heavily saturated.
Instantaneous overcurrent elements are commonly used in protective relays for power systems. The instantaneous current magnitude is compared against a threshold value for fault determination on the preselected line. Overcurrent elements which are implemented in digital form in microprocessor-based protective relays typically use a pair of orthogonal finite impulse filters to properly compute the complex number which represents the magnitude of the fundamental component phasor of the line current. Fourier filters and cosine filters are commonly used examples of such filters. Using such a filter to obtain magnitude values, the fundamental phasor I of a time varying current waveform i(t), with period T, is sampled at a rate of N samples per cycle in accordance with the following formula:   I  =            2      N        ⁢                  ∑                  n          =          0                          N          -          1                    ⁢                        i          ⁡                      (            n            )                          ⁢                  ⅇ                      j            ⁡                          (                              2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                                  n                  /                  N                                            )                                          
The magnitude of the current waveform is then obtained by taking the absolute value of the fundamental phasor. The most typical values of N are 8, 12, 16 or 24 samples-per-cycle.
The full cycle Fourier filter, however, is subject to the effect of an exponentially decaying DC offset, which may exist in the current waveform. This disadvantage of the full cycle Fourier is overcome by using a variation of the Fourier full cycle filter consisting of the cosine part of the exponential value shown in the above equation and then processing the waveform through a xc2xd cycle delay filter. This is typically known as a cosine filter.
A significant shortcoming of these conventional digital filters used to determine current magnitude, however, is that they will not accurately measure the magnitude of the current waveform when the current obtained from the power line is sufficiently large to undergo saturation in the current transformer. Because of the saturation of the current transformer, samples of the current from the line will have a smaller magnitude value than they would have without saturation. The consequence is that an overcurrent relay using digital filters will have a tendency to underreach, i.e. determine a smaller current value than the true value, because of CT (current transformer) saturation. This can result in a significant delay in the tripping action. In cases of extreme saturation, the element could completely fail to operate during a fault, which of course is quite undesirable.
The present invention is designed to remedy this disadvantage of existing overcurrent elements, such that they will properly operate even when the line current heavily saturates the transformer.
Accordingly, the present invention comprises: an instantaneous overcurrent element for use in a microprocessor-based protective relay for a power system, comprising: a digital filter circuit, responsive to a secondary current waveform from a current transformer which is responsive to a current waveform from the power line, for determining the magnitude of the current waveform; a peak detector circuit responsive to the secondary current waveform from the current transformer for determining the peak magnitude of the current waveform; a circuit for determining the distortion of the secondary waveform from the current transformer; a comparison element for comparing the distortion against a predetermined standard; and a switch connecting the output of the filter circuit to a fault determination circuit when the distortion is less than the predetermined standard and connecting the output of the peak detector to the fault determination circuit when the distortion is above the predetermined standard.